It's fine by me to jump in.
Don't let it confuse you too much. Other than feeding layer too early, it is hard to mess this up.
Other than slight differences in vitamins and minerals, the only real difference between feeds is protein % and calcium %.
Start & Grow is 18% protein. Likely the grower feed is also 18% protein.
If grower is 18% there's really no difference other than the size of the crumbles.
If it is 16%, that may make a difference. Young birds need more protein. As they approach full size, they don't need as much.
Be careful with scratch and treats that may cut the overall protein% too much.
I like to stick with 50# bags cause they're cheaper per pound.
For the OP, almost every bag of feed has a chart on the back with all the manufacturers feeds listed and the ages appropriate for each product. If you had read that chart, you would have chosen the right product initially.